1. Field of the Invention
The invention relates generally to the field of computer systems and, more specifically, to a technique for recovering from a single socket failure at a host using two independent sockets.
2. Description of the Related Art
Computer systems include host computers that communicate with one another via a network to run network applications. Such applications include earlier text-based applications that provide remote access to computers, electronic mail, file transfers, news groups and chats, as well as more recent multimedia applications such as the World Wide Web, Internet telephony, video conferencing and audio- and video-on-demand. Typically, software is distributed and run on two or more hosts or end systems to realize the application. For example, for a Web application, browser software on the user's host computer communicates with Web server software in a Web server. The processes running on the different hosts communicate with one another by exchanging messages across a network via sockets. The network applications have application-layer protocols that define the format and order of the messages that are exchanged between the processes. Moreover, the application-layer protocols define what actions to take when a message is transmitted or received. For example, HTTP is an application-layer protocol for the Web that defines how messages are passed between a browser and a Web server. SMTP is an application-layer protocol used for electronic mail.
In particular, a network application typically includes a client side and a server side. In this case, the application may be referred to as a client/server application. A client side on an end host may communicate with a server side on another host. The client is usually the host that initiates a communication or session with another host. In some cases, a host can implement both the client and server sides of an application. For example, a mail server can run the client side of SMTP for sending mail, as well as the server side of SMTP for receiving mail. In this case, the client-side host opens two sockets—one for sending mail and one for receiving mail. In other cases, a host can implement two sockets that are independent. For example, in client/server applications where communications are initiated by a client and a server independent of each other, two communication sockets at each host can be used to simplify the communication. However, this complicates error recovery when there is a socket failure since each socket can fail independently, without the other socket knowing. For example, it is not known if an individual socket has failed or if the entire connection has been lost. Recovery from these scenarios is also problematic.